Electromagnetic interference (EMI), which may also be called radio frequency interference (RFI) in some instances, is a disturbance that affects an electrical circuit due to undesired electromagnetic conduction or electromagnetic radiation. For example, if an electrical circuit in a printer processes 850 kHz signals and nearby wireless transmitter transmits a competing 850 kHz wireless signal, the printer may experience significant signal degradation and possibly make printing mistakes because the wireless transmitter generates EMI that interferes with the printer's own internal signal processing.
To keep EMI within manageable levels, the Federal Communications Commission (FCC) in the United States and other regulatory agencies around the globe have promulgated regulations to establish acceptable EMI levels for electronic devices. Generally speaking, such regulations setup different classes of electronics devices, and assign a maximum EMI level that can be produced by devices within each class. In this way, consumers and businesses can have confidence that their electronic devices should function adequately without being concerned about interference from other devices.
One particularly problematic source of EMI is power supplies for electronic devices. These power supplies often convert power in one format to another format. For example, because some laptop computers include integrated circuits designed to operate on a DC voltage at 19.5 V (and because typical residential and commercial power outlets deliver an AC voltage at 60 Hz and 120 V); laptops often come with a power adapter that converts the AC voltage to a 19.5 V DC voltage, which is suitable for powering the laptop.
In many implementations, these power supplies include one or more switching elements that operate according to a pulse width modulated control signal, thereby maintaining an output power required for a given electronic device. However, because pulse width modulated switches operate at a relatively high frequency compared to the frequency of AC voltage, they can generate a high frequency signal that can cause conductive or radiative EMI problems. With regards to conductive EMI, the high frequency signals can be injected back into the AC mains and become an undesirable component of the AC mains signal. With regards to radiative EMI, the high frequency signals can also be radiated by the power supply as electromagnetic waves. In either case, the EMI generated by the power supply can cause problems for communications devices in the vicinity of the power supply.
Because power supplies generate a major component of the EMI for electronic devices, an important step in designing a power supply is limiting the EMI provided by the power supply to levels with the acceptable limits of the various standards. Therefore, the inventors have devised power supplies that exhibit favorable EMI characteristics while at the same time delivering power to an electronics device in a suitable manner.